As expectations are raised for MEMS and nanotechnology to be a next-generation technology, an attention has been paid to a processing method using interfered laser beams as a method of manufacturing microstructures to be incorporated in the MEMS and involved in the nanotechnology. The processing by the interfered laser beams is capable of processing an area consisting of several μm to several tens cm at a time, unlike electron and ion beam printing. In addition, the processing by the interfered laser beams does not require use of a mask, unlike the photolithography or a LIGA process. Further, in recent years, there has been proposed a processing method using a femtosecond laser as a method of processing a submicron structure using a pulsed laser. The fetmosecond laser has a pulse width of 1 ps (10−12 seconds) or less, and very short in the thermal diffusion length in a case where the laser is irradiated onto a member, thus reducing a damage due to the heat.
Also, the use of the femtosecond laser makes it possible to process a material such as glass or plastic, which normally does not have absorbance at the wavelength of the laser, with resolution at the order of submicron. When those transparent materials are irradiated with the femtosecond laser, there locally occurs absorption of photons, which is called “multiphoton absorption”. The multiphoton absorption is a photon phenomenon that is caused when a material is irradiated with a strong electromagnetic field, in which photons can be absorbed only in a region where a fluence of the irradiated laser is sufficiently strong.
As a conventional example of the processing method using the femtosecond laser, for example, Japanese Patent Application Laid-Open No. 2001-236002 has proposed a production method of a hologram using the interfered femtosecond laser. In this method, fundamental waves (800 nm in wavelength) of the femtosecond laser are caused to interfere with each other to be irradiated onto a surface of glass, to thereby produce a hologram of micro to submicron pitches by ablation. Also, Journal of Nanoscience and Nanotechnology, 2002, Vol. 2, No. 3/4, 321 to 323 has reported as regards the production of a grating of a pitch of 290 nm with respect to a glass surface through interference of triple waves (290 nm in wavelength).
However, in the processing method using the interfered femtosecond laser in the above conventional example, there is such a problem that the processing is distorted by an electromagnetic wave that propagates on a surface of a material to be processed (hereinafter referred to as “surface wave”), i.e., a surface wave that propagates on the surface of the material to be processed in a direction of interference of the laser. The surface wave is a phenomenon in which a light beam scattered due to nonuniformity of the material surface propagates along the surface of the material, which leads to the processing disturbance generically called “ripple” having cyclicality. The above surface wave or the ripple phenomenon occurs not only in the processing using the femtosecond laser, but also in every processing of irradiating a light having a interference property, such as the modification of physicality through exposure, ablation, photo etching, or light illumination. Also, the surface wave or the ripple phenomenon occurs on a surface of every material, such as metal, semiconductor, glass, plastic, or other dielectric materials. However, the conventional laser that is long in the pulse width or a continuous light is largely affected by heat, and the generated ripple configuration is frequently flattened by thermofusion. However, the disturbance of the processing due to the ripple remarkably appears without being subjected to the flattening action by heat in the processing conducted by the femtosecond laser which is very short in the thermal diffusion length and small in the thermal influence as described above. Also, in the case of metal, because the surface wave propagates as a plasma wave through electrically conductive electrons, the occurrence of ripples is remarkable as compared with the case of the dielectric material.
Hereinafter, a description will be further given of the occurrence of ripples. FIGS. 4A and 4B shows an SEM image of ripples that occur in laser irradiation gas etching (refer to Jpn. J. Appl. Phys. Vol. 31 (1992) pp. 4433 to 4436). FIG. 4A shows a linear polarization having an amplitude of an electric field in a direction indicated in FIG. 5A, that is, ripples caused by an incidence of p-polarized beam. FIG. 4B shows a linear polarization having an amplitude of an electric field in a direction indicated in FIG. 5B, that is, ripples caused by an incidence of S-polarized beam. When it is assumed that the number of waves of the laser is k0, the number of surface waves is kSEW, and an incident angle is θ, pitches dr of the ripples in P-polarization and in S-polarization are expressed by the following expressions (1) and (2), respectively. More specifically, k0=2.37×10−5 cm−1 (266 nm in the wavelength), θ=13.5°, and kSEW=2.55×10−5 cm−1. Therefore, dr (P-polarization)=345 nm, and dr (S-polarization)=265 nm.
                              d          r                =                                            2              ⁢              π                                                      k                SEW                            -                                                k                  0                                ⁢                sin                ⁢                                                                  ⁢                θ                                              ⁢                                          ⁢                      (                          P              ⁢                                                          ⁢              deflection                        )                                              (        1        )                                          d          r                =                                            2              ⁢              π                                      k              SEW                                ⁢                                          ⁢                      (                          S              ⁢                                                          ⁢              deflection                        )                                              (        2        )            
The above ripples are not limited to a case in which the ripples are provided with one cyclicality in one direction as shown in FIGS. 4A and 4B. FIG. 6 shows an SEM image of ripples that occur when diamond is subjected to ablation processing by the femtosecond laser (refer to Applied Physics Letters, Volume 82, No. 11. (2003) p. 1703 to 1705). In this case, three kinds of ripples different in the pitch and direction from one another can be recognized.
FIGS. 7A and 7B show ripples that occur when the femtosecond laser is perpendicularly irradiated onto a surface of a nickel member at a right angle, and the nickel surface is subjected to ablation processing. As in the case of diamond, three kinds of ripples can be recognized. Those ripples occur because the surface wave having three different wavelengths propagate in a direction orthogonal or parallel with respect to the polarization. Because the incidence angle of the laser is θ°, the number (kSEW) of surface waves is obtained from the measured pitch (dr) of the ripples by the following expression (3) on the basis of the expressions (1) and (2).
                              d          r                =                              2            ⁢            π                                k            SEW                                              (        3        )            
Table 1 collectively shows a direction of ripples with respect to the polarization which occur in FIGS. 7A and 7B (that is, a propagation direction of the surface wave), the pitch (dr) of the ripples, the number of surface waves (kSEW) which is obtained from the pitches (dr) and the expression (3), and the like. In Table 1, the ripples of nickel are expressed as ripple 1, ripple 2, and ripple 3 in ascending order of the number of waves thereof.
TABLE 1Ripple 1Ripple 2Ripple 3DirectionOrthogonalParallel toOrthogonalof ripplesto thetheto the(propagationpolarizationpolarizationpolarizationdirectionof surfacewave)Periodicity1940 nm730 nm120 to 430 nmof RippleNumber of3.24 × 10−38.60 × 10−314.6 tosurface waves(1/nm)(1/nm)52.3 × 10−3(1/nm)
The above three kinds of ripples are closely associated with polarization of the laser. FIG. 12 shows an SEM image of the nickel surface in a case where an interfered femtosecond laser is irradiated onto the surface of nickel to produce a grating of the same pitch with that of the interference by ablation. The wavelength of the used femtosecond laser is 800 nm, the interference angle is 90°, and the pitch of interferences is 560 nm. A polarization of the laser is perpendicular to a surface including two laser beams that interfere with each other (hereinafter, called “S-polarization interference”). FIG. 13 is a schematic diagram showing an S-polarization state. In the S-polarization state, the polarization of the two laser beams always coincide with each other regardless of the interference angle. Accordingly, as shown in FIG. 14, the S-polarization state is a state in which a largest difference between a peak 16 (abdomen) and a bottom 17 (node) of the interference, that is, the highest contrast of interference, is obtained, and the S-polarization state is generally used in a process using the interference. As is apparent from FIG. 12, a large number of ripples occur on the surface of nickel which has been processed, and those ripples hinder production of an intended grating of 560 nm in pitch.
As a method of eliminating the above disturbance and breakdown of processing due to the ripples (surface waves), a method is proposed in which a circularly polarized or an ellipsoidally polarized light is used. The use of a polarization state that is attributable to the circularly polarized light or ellipsoidally polarized light makes it possible to rotate the direction in which the ripples occur, along with the rotation of the polarization, to apparently cancel the ripples. However, in a manner of removing the ripples according to the above method, the pitches of ripples are merely made unrecognized through the rotation of the ripples, and a region to be processed is remarkably larger than the case in which processing is conducted by linear polarization. Accordingly, the above method cannot be substantially employed in processing of a submicron size. Also, the processing using the circular polarization or the ellipsoidal polarization cannot be used in an optical system which is greatly affected by the polarization, such as an imaging optical system using the interference or a phase mask.
The present invention has been made in view of the above problems, and therefore has an object to provide a processing method and a processing apparatus which are capable of suppressing a disturbance attributable to a surface wave that occurs in such processing as ablation processing, modification of a material surface or the exposure of a resist, using a interfered laser, in particular, a process using a interfered laser of a pulsed laser having a pulse width of equal to or more than 1 fs and of equal to or less than 1 ps.